The user that uploaded that theme could have a differen't operating system than you.

Are you using win vista, 7 or 8?

There maybe a way to edit the gimp theme file(using notepad or notepad++), but it looks like the tabs are adapting your system's theme.

Here are a few things you can also try (assuming you're on windows)
(each number is an individual thing to try, not a step*)
1
Find your "Performance options" and under the "Visual Effects" tab
-look for any options to use system styles if any. Try checking it off/on (what OS are you using?)

2
On your gimp.exe program shortcut, right-click it & go to "Properties"
Then click the "Compatibility" tab
-- Check if you have a []Disable visual theme , check that on if you can.
(Mine had that option on XP, but not win8...)

3
You could try changing the theme of your system.
- Right-click desktop "Personalize" look for basic theme, try that if you aren't on it already.
It maybe as simple as colors or complex as the gUi

tar.gz is very much linux. I get the same result in a Win7 VM, the tabs and borderes are white. Going into the Windows theme settings and I can see no way of changing those.
If it is any consolation in my pclinuxos, the tabs are dark but the background to the tools is white - ie. equally screwed up.

One way of getting your dark theme. Go to http://www.partha.com, get the appropriate 32/64 bit Win installer and reinstall Gimp. Partha's version has a dark theme as standard (regular theme is there as well, for dark theme haters = me).

Pop that Gimp-CS6 theme folder (without the pdf, and Setting etc folder) into your users/.../.gimp-2.8/themes folder
Select it from Edit -> Preferences -> themes.
Restart Gimp for it to take effect.